1. Field of the Invention
The present invention relates to a composite substrate, a surface acoustic wave device, and a method for manufacturing a composite substrate.
2. Description of the Related Art
Composite substrates having a structure in which a supporting substrate and a piezoelectric substrate are bonded to each other are known. Such composite substrates are used, for example, for surface acoustic wave devices in which a comb-shaped electrode capable of exciting a surface acoustic wave is disposed on a surface of a piezoelectric substrate. In the case of a composite substrate in which a piezoelectric substrate and a supporting substrate are bonded to each other at bonding surfaces which are mirror surfaces, reflection of bulk waves occurs at the interface because of a difference in acoustic impedance between the piezoelectric substrate and the supporting substrate. Reflected bulk waves reach the surface of the piezoelectric substrate again and are detected as signals by the electrode. Reflection of unwanted bulk waves in such a manner will degrade the characteristics of the surface acoustic wave device. As one of countermeasures against this problem, by roughening the back surface of the piezoelectric substrate, i.e., the surface in contact with the supporting substrate, bulk waves are scattered so as to prevent the degradation of the characteristics. For example, Patent Literature 1 describes that, in a surface acoustic wave element having a structure in which a piezoelectric substrate and a supporting substrate are bonded to each other by an adhesion layer, by forming fine irregularities on a surface (back surface) of the piezoelectric substrate to be bonded to the supporting substrate, generation of spurious signals can be suppressed.